Multiple access schemes are employed by modern wireless communications systems to allow multiple users to share a common limited spectrum or bandwidth resources, while maintaining acceptable system performance. Common multiple access schemes include Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). Traditionally this resource sharing has been achieved in an orthogonal way, meaning that the users do not interfere with each other as they transmit their signals. The two most common orthogonal multiple access techniques are time TDMA where the users take turn in using the whole spectrum (via time slots), and FDMA where the users divide the spectrum among themselves and transmit continuously. Most often, a combination of the two methods is used. Non-orthogonal multiple access (NOMA), on the other hand, allows users to interfere with each other, and sorts them out at the receiver. Various receiver algorithms exist with varying complexity/performance trade-offs.
System performance is also aided by error control codes or forward error correction (FEC) encoding. Nearly every communications system relies on some form of error control for managing errors that may occur due to noise and other factors during transmission of information through a communications channel. These communications systems can include satellite systems, fiber-optic systems, cellular systems, and radio and television broadcasting systems. Efficient error control schemes implemented at the transmitting end of these communications systems have the capacity to enable the transmission of data (including audio, video, text, etc.) with very low error rates within a given signal-to-noise ratio (SNR) environment. Powerful error control schemes also enable a communications system to achieve target error performance rates in environments with very low SNR, such as in satellite and other wireless systems where noise is prevalent and high levels of transmission power are costly, if even feasible. A broad class of powerful error control schemes that enable reliable transmission of information have emerged, including low density parity check (LDPC) codes and turbo codes. Both LDPC codes as well as some classes of turbo codes have been successfully demonstrated to approach near the theoretical bound (i.e., Shannon limit). Although long constraint length convolutional codes can also approach the Shannon limit, decoder design complexity prevents practical, wide spread adoption. LDPC codes and turbo codes, on the other hand, can achieve low error rates with lower complexity decoders. Consequently, these codes have garnered significant attention.
For example, conventional data transmission to and from an ultra-small terminal via satellite is usually based on Code Division Multiple Access (CDMA) technique, for example using FEC encoding at high code rates such as 1/2 or 1/3 (e.g., turbo codes or LDPC codes). CDMA spreads bandwidth to reduce the interference between adjacent satellites, whereas the error coding provides coding gain needed to close the link. CDMA also allows multiple users sharing common bandwidth at the same time. CDMA systems, however, typically need a large bandwidth expansion factor to function properly. Additionally, CDMA systems require all signals accessing the same spectrum at the same time to be of equal power, and provision for power control makes CDMA system more complicated to implement. Traditionally, multiple-user access systems (e.g., NOMA systems) employ error coding that provides the best performance for a single-user system access system (where the same bandwidth resources are not shared by multiple users). Moreover, based on different requirements and regulations (e.g., set by Federal Communications Commission (FCC), International Radio Union, etc.), limit various radio transmission attributes of terminals, such as antenna side lobe, power density at antenna flange, off-axis effective isotropic radiate power (EIRP) density, etc., for example, radiated by terminals that communicate via satellite. Accordingly, in order to provide uplink closure at high data rates using small aperture antenna (for example, in small terminals), such regulatory limits can easily be exceeded by conventional radio transmission means.
Accordingly, there is a need for a multiple access scheme (e.g., a NOMA scheme) combined with a forward error correction (FEC) encoding scheme that can effectively utilize a broad range of code rates for different terminal types in differing channel environments.